At Night, I Become a Monster novel features story by Yoru Sumino.

Every night, Adachi transforms into a nightmarish creature- and every morning, he reverts to human form. When he encounters his bullied classmate Yano Satsuki in his monstrous state, the two develop a peculiar bond. But daylight brings its own form of terrors. Which is worse, the monster at night or the cruel realities of the classroom by day?